Prion protein (PrPC) is a normal glycoprotein implicated in the pathogenesis of prion disorders, a group of fatal neurodegenerative conditions of humans and animals. A change in the conformation of PrPC to an aggregated, PrP-scrapie (PrPSc) form is believed to be the principal cause of neurotoxicity in these disorders. Recently, we demonstrated that PrPC plays a functional role in cellular iron uptake and transport, and selective deletion of PrPC in PrP knock-out (PrPKO) mice induces a state of systemic iron deficiency in these animals relative to wild-type (wt) controls. Specifically, PrPKO mice show impaired transport of orally introduced iron from the duodenum to the blood stream, and inefficient uptake of plasma iron by hematopoietic precursor cells and parenchymal cells of major organs. Together with our recent report demonstrating a phenotype of iron deficiency that correlates with PrPSc deposits in prion disease affected brains, these observations suggest that loss of normal function of PrPC due to aggregation to the PrPSc form may be responsible for brain iron dys- homeostasis in diseased brains. In this application, we will focus on the mechanism of iron modulation by PrPC, and hypothesize that PrPC is a novel iron uptake and transport protein, and modulates cellular iron metabolism either directly or by interacting with other iron transport protein(s). The proposed studies will test this hypothesis using two complementary approaches: 1) by evaluating the transport of different sources of iron that utilize distinct pathways of transport in the same tissue, and 2) by investigating the transport of same source of iron across tissues that utilize distinct pathways of uptake and transport. Mouse models that express no PrPC (PrPKO), wt levels (wt), and 10-fold higher than wt levels of PrPC (PrPOV) will be used for this analysis. This approach will allow identification of pathways of iron transport by PrPC, and the point where PrPC intersects with known pathways of iron metabolism. Three specific aims are proposed to accomplish these goals. In aim 1, the functional role of PrPC in brain iron metabolism will be evaluated in wt, PrPKO, and PrPOV mice, and the underlying mechanism will be investigated in neuroblastoma cells expressing normal and mutant forms of PrP defective in iron transport. In addition, the correlation between PrPC, PrPSc, and brain iron status will be assessed in scrapie infected wt and PrPOV mice. In aim 2, the transport of different sources of iron will be checked in hematopoietic and reticuloendothelial cells isolated from wt, PrPKO, and PrPOV mice, and the underlying mechanism will be investigated in K562 erythroleukemia cells transfected with normal and mutant PrP forms. In aim 3, the uptake and transport of different sources of iron across the intestinal epithelium will be checked in wt, PrPKO, and PrPOV mice, and compared with aim 2 above. Based on in vivo results, polarized Caco-2 cells transfected to express PrPC or mutant PrP forms will be used as models of absorptive enterocytes to understand the mechanism of iron transport by PrPC. Successful completion of these studies will uncover novel pathway(s) of iron modulation by PrPC, and improve our understanding of the mechanism(s) underlying brain iron imbalance and associated neurotoxicity in prion disorders.